The invention relates to a current limit circuit and more specifically to a temperature compensated current limit circuit implemented in a solid state switch.
Voltage regulation integrated circuits provide a regulated output voltage to a load. These circuits often include a current limitation feature that prevents current in excess of a predefined limit from flowing in the integrated circuit if the load increases to an unacceptable level. Another class of circuits that rely on accurate current limit protection are solid state switch circuits. These circuits provide a low impedance connection between two nodes and limit current to less than a predetermined value. Without compensating for temperature variations, a shift in the current limit value can occur. For example, the current limit value can shift by more than forty percent over a range of xe2x88x9240xc2x0 C. to +85xc2x0 C. due to temperature dependent electrical characteristics of the materials used in the circuit components. Thus, the current limiting portion of the circuit may not provide adequate protection in certain applications. What is needed is a circuit that provides a stable current limit over a wide temperature range.
The present invention relates to a circuit and a method of stabilizing the current limit over a range of temperatures. The present invention is directed to limiting the amount of current flow through a metal interconnect, thus providing temperature current limit protection.
One aspect of the invention relates to a temperature compensated current limit circuit used in a solid state switch. The circuit, under normal operating conditions, fully enhances an integrated MOSFET, resulting in a reduced voltage drop across the switch. When the load current increases to an unacceptable level, the switch limits the current delivered to a load via a servo loop. The current delivered to a load without exhibiting any current limit behavior is often referred to as current compliance.
The circuit includes a first resistive element, a second resistive element, a load current controller, a current module, and an amplifier. The first resistive element has a first terminal adapted to receive an input voltage and has a second terminal. The first resistive element has a temperature dependent resistivity. The second resistive element has a first terminal configured to receive the input voltage and has a second terminal. The load current controller has a first terminal in communication with the second terminal of the first resistive element, a second terminal in communication with a load, and a control terminal adapted to receive a control signal. The amplifier has a first input terminal in communication with the second terminal of the first resistive element, a second input terminal in communication with the second terminal of the second resistive element, and an amplifier output terminal in communication with the control terminal of the load current controller. The amplifier provides the control signal at its output terminal in response to a load current, the resistivity of the first resistive element, the resistivity of the second resistive element, and a temperature dependent current generated by the current module. The current module has a first terminal in communication with the second terminal of the second resistive element and the second terminal of the amplifier. The current module provides a temperature dependent current at its first terminal.
In one embodiment, the load current controller is a current controlling transistor. In another embodiment, the current module generates a current that is proportional to absolute temperature (PTAT). In still another embodiment, the second resistive element includes a primary resistive element, and a secondary resistive element, each having first and second terminals. The first terminal of the primary resistive element is in communication with the first terminal of the second resistive element. The first terminal of the secondary resistive element is in communication with the second terminal of the primary resistive element. The second terminal of the secondary resistive element is in communication with the second terminal of the second resistive element. In a further embodiment, the resistivity of the primary resistive element is greater than the resistivity of the secondary resistive element.
Another aspect of the invention relates to a method of providing a voltage across a load with a temperature independent current compliance. The method includes the steps of generating a first temperature dependent voltage drop in response to the current through the load, generating a second temperature dependent voltage drop in response to a temperature dependent reference current, and amplifying the difference of the first temperature dependent voltage drop and the second temperature dependent voltage drop to generate a control signal. Additionally, the method includes the step of applying the control signal to a load current controller to provide the voltage having a temperature independent current compliance across the load. The method can be applied repeatedly to achieve a continuing current limitation function.
In another aspect, the method includes the steps of comparing a first temperature dependent voltage drop to a second temperature dependent voltage drop, wherein the second temperature dependent voltage drop is responsive to a temperature dependent current, generating a control signal in response to the comparison, and generating the voltage having a temperature independent current compliance in response to the control signal.